Elements essential for metabolism of plants provide the power and building blocks, which are required for sustaining growth of plants and completion of life cycle. Such elements may be added to increase growth and yield. Another group of compounds, which also occur in nature, may be supplemented to plants in order to manipulate the growth and the development of the plant. This group is composed of plant growth regulators (PGR) or phytohormones also known as plant hormones. Since naturally occurring PGR are sensitive to ex situ breakdown and to metabolism in situ, they were frequently replaced by synthetic PGR for practical applications mostly in agriculture.
Plant hormones play a crucial role in controlling the way which plants grow and develop. While metabolism provides the power and building blocks for plant life, it is the hormones that regulate the speed of growth of the individual parts and integrate these parts to produce the form that we recognize as a plant, e.g. the branching of the plants. In the plant world several classes of hormones are of major importance {for general literature see: (a) Davis P J, ed., ‘Plant Hormones’ Physiology, Biochemistry and Molecular Biology, Kluwer Academic Publishers, Dordrecht, Boston, London, 1995; (b) George E F, Plant Propagation by Tissue Culture. Part 1, The Technology; Part 2, In practice, Exegetics Ltd., Edington, 1993, 1995; (c) Basra A. S. ed, Plant Growth regulators in Agriculture and Horticulture. Food products Press. New York 2000; and (d) Taiz L. and Ziegler E. Plant Physiology 3rd edition. Sinauer Associates Inc Publisher. Sunderland, Mass.}:
Auxins such as indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 1-naplhtlhaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) affect mainly cell division and enlargement and stem growth, vascular tissue differentiation in phloem and xylem, root initiation and callus formation.
Cytokinins such as zeatin (Z), 6-benzylaminopurine (BAP) and kinetin affect mainly adventitious shoot formation, cell division and inhibition of root formation.
Gibberellins such as gibberellic acid (GA3), gibberellin 1 (GA1), gibberellin 4 (GA4) affect mainly stem growth, bolting in long day plants and release from dormancy in seeds, somatic embryos, apical buds and bulbs.
Ethylenes have an effect on shoot and root growth and differentiation, root formation, abscission of leaves and roots, flower and leaf senescence and fruit ripening.
Abscisic acids have an effect on stomata closure, inhibition of shoot growth, induction of seed dormancy and production of storage protein.
In recent years more substances, e.g. brassinolides, jasmonic acid and salicylic acid were recognized as plant growth regulators.
In respect to the effect of plant hormones solely on plant development, the most important are the stimulation of auxin and gibberllins on cell elongation and differentiation, the effect of cytokinin, auxin and gibberllins on cell division, and the effect of both auxins and cytokinins on organ differentiation.
Plant hormones have commercial value in agriculture and horticulture. A limited list of applications related to growth stimulation include enhancement of the size of seedless grapes by GA, development of partenocarpic fruits by auxin, root induction in plant propagation by auxins.
One other group of plant growth regulators known as triazoles has a pronounced effect of stress protection. These compounds inhibit GA biosynthesis and also affect the level of other hormones as ABA, cytokinin and ethylene. Triazole-treated plants use less water and have increased tolerance to drought mainly due to reduced transpiration caused by decreased leaf area and increased epicuticular wax accumulation. Under conditions of water deficiency, triazoles have an increased effect on the growth of plants. Triazoles also increase tolerance of plants to chilling and freezing temperatures, to high temperature stress and to air pollutants such as SO2 and O3.